c++: -M and modules again

While experimenting with testing module std I noticed that gcc -M broke on
it; it seems I need to set directives_only even sooner than I did in
r15-4219.

gcc/c-family/ChangeLog:

	* c-ppoutput.cc (preprocess_file): Don't set directives_only here.

gcc/cp/ChangeLog:

	* module.cc (module_preprocess_options): Set directives_only here.